This invention relates to a fluidized bed reactor and method for operating same and, more particularly, to a fluidized bed reactor utilizing an improved system for removing particulate material from the reactor bed.
Reactors, such as combustors, steam generators and the like, utilizing fluidized beds as the primary source of heat generation are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel, such as coal, and an adsorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at relatively low temperatures. When the reactor is utilized as a steam generator, the heat produced by the fluidized bed is utilized to convert water to steam which results in an attractive combination of high heat release, high sulfur adsorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed combustion system is commonly referred to as a "bubbling" fluidized bed in which a bed of particulate material is supported by an air distribution plate, to which combustion-supporting air is introduced through a plurality of perforations in the plate, causing the material to expand and take on a suspended, or fluidized, state. The gas velocity is typically two to three times that needed to develop a pressure drop which will support the bed weight (e.g., minimum fluidization velocity), causing the formation of bubbles that rise up through the bed and give the appearance of a boiling liquid.
In an effort to extend the improvements in combustion efficiency, pollutant emissions control, and operation turn-down afforded by the bubbling bed, a fluidized bed reactor has been developed utilizing a "circulating" fluidized bed. In these arrangements the mean gas velocity is increased above that for the bubbling bed, so that the bed surface becomes more diffused and the solids entrainment from the bed is increased. According to this process, fluidized bed densities are attained which are well below those typical of the bubbling fluidized bed. The formation of the low density circulating fluidized bed is due to its small particle size and to a high solids throughput, which require high solids recycle. The velocity range of a circulating fluidized bed is between the solids terminal, or free fall, velocity and a velocity beyond which the bed would be converted into a pneumatic transport line.
U.S. Pat. Nos. 4,809,623 and 4,809,625, assigned to the same assignee as the present application, disclose a fluidized bed reactor in which a dense, or bubbling, bed is maintained in the lower portion of the furnace, while the bed otherwise is operated as a circulating bed. The design is such that advantages of both a bubbling bed and a circulating bed are obtained, not the least significant advantage being the ability to utilize particulate fuel material extending over a greater range of particle sizes.
In these designs a homogenous mixture of fuel particles and adsorbent particles (hereinafter collectively referred to as "particulate material") is formed, with a portion of the fuel particles being unburned, a portion being partially burned and a portion being completely burned and a portion of the adsorbent being unreacted, a portion being partially reacted and a portion being completely reacted. The particulate material must be discharged from the system quickly and efficiently to accommodate the continuous introduction of fresh fuel and adsorbent. To this end, a portion of the particulate material is usually passed from the lower portion of the bed to one or more stripper/coolers located adjacent the furnace section of the reactor. Air is blown through the stripper section of the stripper/cooler to entrain some of the relatively fine particulate material which is returned to the furnace. The remaining particulate material in the stripper/cooler is passed to its cooler section and water/steam is passed in a heat exchange relation to the latter material to remove heat from the material before it is discharged from the system.
However, in some situations, such as when fuels that generate a lot of relatively fine ash are used, or when a relatively large amount of relatively fine adsorbent has to be used with fuels having a relatively high sulfur content, the relatively fine particle material stripped in the stripper/cooler and returned to the furnace section increases the volume of the fines, or the "loading" in the upper furnace section of the reactor, to unacceptable levels. This requires large and expensive stripper/coolers and/or requires that the furnace be operated at low stoichiometry, which is inefficient. Also, these stripper/coolers cannot handle very large amounts of relative coarse material. Thus, these prior art stripper/coolers limit the range of particle sizes that can be used to maintain adequate efficiency.